An aerogel is a superporous, high specific surface area (500 m2/g) material having a porosity of about 90 to 99.9% and a pore size in the range of 1 to 100 nm, and is a material excellent in ultra-light weight, super thermal insulation, ultra-low dielectric, and the like. Accordingly, research on the development of aerogel materials as well as research on the practical use thereof as transparent insulation materials, environmentally friendly high temperature insulation materials, ultra-low dielectric thin films for highly integrated devices, catalysts and catalyst carriers, electrodes for supercapacitors, and electrode materials for seawater desalination have been actively studied.
The biggest advantage of the aerogel is that the aerogel has a super-insulation exhibiting a thermal conductivity of 0.300 W/m·K or less, which is lower than that of an organic insulation material such as conventional Styrofoam, and that fire vulnerability and the occurrence of harmful gases in case of fire which are fatal weaknesses of the organic insulation material can be solved.
In general, the aerogel is produced by preparing a hydrogel from a silica precursor such as water glass and alkoxysilane series (TEOS, TMOS, MTMS, etc.), and removing a liquid component inside the hydrogel without destroying a microstructure. The typical form of a silica aerogel may be classified into three types, i.e., powder, granule, and monolith, and the silica aerogel is generally produced in the form of powder.
Meanwhile, when the silica aerogel absorbs moisture, the characteristics and physical properties of a gel structure are deteriorated. Therefore, in order to easily use the silica aerogel in industries, a method which is capable of permanently preventing moisture in the air from being absorbed is required. Accordingly, methods for producing a silica aerogel having permanent hydrophobicity by hydrophobizing the surface thereof have been proposed.
Accordingly, the silica aerogel is generally produced by a sol-gel method in which sol formation, hydrogel formation, aging, solvent exchange, surface modification, and drying are carried out. However, the sol-gel method requires a very complicated process and requires much cost and time, thus deteriorating the productivity and economical efficiency of the silica aerogel. Therefore, the development of a novel method for producing a silica aerogel having better physical properties by a simpler process is required.
In addition, when an ambient drying technique is used to ensure price competitiveness, a pore structure is shrunk due to a high capillary force. Thus, it is difficult to predict a tap density of the finally produced silica aerogel, and also difficult to produce the silica aerogel according to the desired specific tap density. Therefore, it is necessary to develop a method for producing a silica aerogel the density of which is controllable.